1. Field of the Invention
The present invention relates to an optical member, a method for manufacturing silica glass, and a reduction projection exposure apparatus; and more particularly, it relates to an optical member made of silica glass suitable for use in a reduction projection exposure apparatus or the like utilizing vacuum ultraviolet light such as an ArF excimer laser, a method for manufacturing the silica glass, and a reduction projection exposure apparatus using the optical member.
2. Related Background Art
Conventionally, a reduction projection exposure apparatus (photolithography apparatus) such as a stepper is used in a process of transferring a fine pattern of an integrated circuit onto a wafer of silicon or the like in case where ICs (Integrated Circuits) and LSIs (Large Scale Integrated Circuits) are manufactured. For this reduction projection exposure apparatus wider exposure areas and higher resolution over that entire exposure area have been required and improvement of the apparatus has been researched by shortening the wavelength of light emitted from the exposure light source or by increasing the numerical aperture (NA) of the projection optical system or the like. Particularly shortening the wavelength of light is very effective in improving the resolution of the apparatus which causes the shift from a g-line (436 nm) to an i-line (365 nm), farther to a KrF excimer laser (248 nm) or an ArF excimer laser (193 nm).
On the other hand, for optical members making up an optical system of a reduction projection exposure apparatus, high light transmission property and ultraviolet light resistance are required suitable for the usage of light having such a short wavelength. For example, optical members used in a projection optical system of a stepper are required to have high light transmission property with internal absorption loss coefficient of 0.001 cmxe2x88x921 with respect to light having a short wavelength as described above, that is a light absorption amount of 0.1% or less per 1 cm of thickness.
However, conventional optical glass used in an optical system of a reduction projection exposure apparatus utilizing a g-line or an i-line doesn""t have a sufficient transmission property with respect to light of which the wavelength is shorter than that of the i-line and exhibits little transmittance with respect to light having a wavelength of 250 nm or less. Therefore, an optical member using silica glass or calcium fluoride crystal which exhibit higher light transmission property with respect to light having such a short wavelength has been under development.
As for a synthesis method of said silica glass, a vapor phase synthesis method called the direct method utilizing silicon tetrachloride as the materials is used conventionally. In this method, first, by ejecting high purity silicon tetra chloride gas as a material from the central portion of a burner made of silica glass with a multi-tubular structure and by ejecting oxygen gas and hydrogen gas from the peripheral portion of a material ejecting opening, fine particles of silica glass are synthesized through a hydrolytic reaction between silicon tetrachloride and the water which is generated through burning of oxygen and hydrogen. And the fine particles of silica glass are deposited on a target rotating, rocking and moving downwardly under said burner, which is fused and vitrified by combustion heat of the hydrogen gas to obtain a silica glass ingot. Silicon tetrachloride as a material is generally ejected by diluting with a carrier gas and in many cases oxygen gas is utilized as the carrier gas. As for the target for depositing the fine particles of silica glass, the one made of an opaque silica glass board is usually used.
However, the above mentioned method has the disadvantage that hydrogen chloride is generated at the time of synthesizing, and optical members made of silica glass obtained by this method have problems in regard to ultraviolet light resistance such as significant lowering of light transmission property when they are irradiated with ultraviolet light of high output or excimer laser beam for a long period of time. This lowering of light transmission property is due to the appearance of an absorption band of 5.8 eV called the Exe2x80x2 center, which is considered to be induced by chloride remaining in the silica glass in a concentration of from 30 to 150 ppm.
Examination into a method for manufacturing silica glass using organosilicon compound which doesn""t contain substantial amounts of chloride as the material for the purpose of improving the above mentioned problems has been being carried out in recent years. However, in the case of silica glass obtained by the method utilizing the organosilicon compound as the material, the internal absorption coefficient for light having a wavelength of 210 nm or less exhibits 0.01 cmxe2x88x921 or more. Therefore, in case such an optical member made of silica glass is used as a lens of an optical system or the like, sufficient resolution cannot be achieved in a reduction projection exposure apparatus or the like utilizing an ArF excimer laser (193.4 nm).
In this way, an optical member made of silica glass with desirable light transmittance and ultraviolet light resistance suitable for use with light having a short wavelength such as the ArF excimer laser hasn""t yet been developed.
Therefore, it is the purpose of the invention to provide an optical member made of silica glass with high light transmission property and ultraviolet light resistance suitable for use in an optical system of a reduction projection exposure apparatus using an ArF excimer laser as a light source or the like, a method for manufacturing the silica glass, and a reduction projection exposure apparatus using the optical member.
As a result of repeated diligent studies to achieve the above described purpose, the present inventors found that formyl radical generated by the exposure of X-rays into silica glass affects the optical quality of the silica glass and it is possible to control the concentration of the formyl radical by selecting conditions such as a ratio between a hydrogen gas and an oxygen gas ejected from a burner and kinds or flow amounts of the materials and the carrier gas in a method for manufacturing silica glass by the direct method. And based on those findings the present invention has been accomplished where silica glass with low concentration of formyl radical generated by the exposure of X-rays can be obtained to solve the above mentioned problems by using such silica glass as material for optical members by allowing the tube placed in the center of the burner with multi-tubular structure to eject an organosilicon compound and an inactive gas and by allowing the tube placed around the tube placed in the center to eject an oxygen gas and a hydrogen gas so that the ratio of the total oxygen gas amount to the total hydrogen gas amount becomes 0.53 or more, so as to allow the above organosilicon compound to react in the oxidizing flame in the method for manufacturing silica glass by the direct method.
Namely, an optical member according to the present invention is:
an optical member made of silica glass synthesized by the direct method where a material gas comprising an organosilicon compound is allowed to react in an oxidizing flame,
said optical member having a 2xc3x971014 molecules/cm3 or less concentration of formyl radical generated by X-ray irradiation whose dose is 0.01 Mrad or more and 1 Mrad or less.
Furthermore, a method according to the present invention is:
a method for manufacturing silica glass comprising:
a first step of allowing an organosilicon compound to react in an oxidizing flame while ejecting an organosilicon compound and an inactive gas from a tube placed in the center of a burner with a multi-tubular structure and ejecting an oxygen gas and a hydrogen gas a tube placed around the tube placed in the center of said burner so that a ratio (a/b) of the total oxygen gas amount (a) and the total hydrogen gas amount (b) being 0.53 or more, so as to obtain silica glass fine particles; and
a second step where said silica glass particles are deposited on a target of heat resistance opposed to said burner and are melted and vitrified, so as to obtain a silica glass ingot.
Moreover, an apparatus according to the present invention is:
a reduction projection exposure apparatus having an exposure light source, a photomask formed with an original image of pattern, an irradiation optical system for irradiating said photomask with the light emitted from said light source, a projection optical system for projecting onto a photosensitive substrate a pattern image projected from said photomask, an alignment system for aligning said photomask and said photosensitive substrate with each other;
wherein at least a portion of the lenses making up said irradiation optical system, the lenses making up said projection optical system and said photomask is an optical member made of silica glass synthesized by the direct method where a material gas comprising an organosilicon compound is allowed to react in an oxidizing flame,
said optical member having a 2xc3x971014 molecules/cm3 or less concentration of formyl radical generated by X-ray irradiation whose dose is 0.01 grad or more and 1 Mrad or less.
According to the present invention, in a method for manufacturing silica glass by the direct method, the residue of carbon in the obtained silica glass is controlled by allowing an organosilicon compound to react in an oxidizing flame while ejecting an organosilicon compound and an inactive gas from the tube placed in the center of the burner with multi-tubular structure and ejecting oxygen gas and hydrogen gas from the tube arranged around the tube placed in the center with a ratio (a/b) of the total oxygen gas amount (a) to the total hydrogen gas amount (b) being 0.53 or more, so that silica glass of which the concentration of formyl radical generated by the irradiation of X-rays is 2xc3x971014 molecules/cm3 or less can be obtained. And by using an optical member comprising silica glass obtained by such a method in an optical system of a reduction projection exposure apparatus, light transmission property and ultraviolet light resistance of the entire optical system can be enhanced against light having a short wavelength such as vacuum ultraviolet light or an excimer laser beam, therefore high resolution can be achieved in the above mentioned apparatus which couldn""t be obtained in the case when an optical member according to the prior art is used.